1. Field of the Invention
The present invention relates to an ink composition which is suitably used for inkjet recording and to an inkjet recording method. More specifically, the present invention relates to an ink composition suitable for inkjet recording, which is cured with high sensitivity upon irradiation with active radiation rays and in which even after the ink has been cured, a cured material thereof has sufficient flexibility, and to an inkjet recording method.
2. Description of the Related Art
Examples of an image recording method for forming an image on a medium to be recorded such as paper on the basis of an image data signal include an electrophotographic mode, a sublimation type and fusion type thermal transfer mode and an inkjet mode. Since the electrophotographic mode needs a process for forming an electrostatic latent image on a photoreceptor drum by means of electrostatic charge and exposure, the system becomes complicated, resulting in high manufacturing costs. Also, in the thermal transfer mode, though an apparatus is inexpensive, an ink ribbon is used, and therefore, not only are the running costs high, but waste is generated.
On the other hand, in the inkjet mode, not only is an apparatus inexpensive, but an ink is discharged into only a required image part, thereby forming an image directly on a medium to be recorded. Therefore, the ink can be efficiently used, and the running costs are inexpensive. Furthermore, a noise is low, and this inkjet mode is excellent as an image recording mode.
As an ink composition which is curable upon irradiation with active radiation rays such as ultraviolet rays (radiation curable ink composition), for example, ink compositions which can be cured with high sensitivity to form an image with high image quality are demanded as an inkjet recording ink composition. By achieving high sensitization, advantages are achieved in that consumed electric power can be reduced and in that a load to an active radiation ray generator is decreased, whereby a high life span can be realized. Furthermore, by achieving high sensitization, since sufficient curing is achieved upon irradiation with active radiation rays, various advantages are achieved such as in that the vaporization of uncured low-molecular weight substances is suppressed and in that a lowering of the strength of a formed image is suppressed.
In recent years, an ultraviolet ray curable inkjet mode is drawing attention in view of the face that the ink has a relatively low odor and is quick-drying and that recording on a non-ink-absorbing medium to be recorded can be achieved. In particular, as disclosed in Bruce M. Monroe, et al., Chemical Reviews, Vol. 93 (1993), pages 435 to 448 and so on, benzil, benzoin, benzoin ethyl ether, Michler's ketone, anthraquinone, acridine, phenazine, benzophenone, 2-ethylanthraquinone and the like have been generally used as a photopolymerization initiator.
However, since photopolymerizable compositions using such a photopolymerization initiator are low in sensitivity to curing, a long time is required for image exposure for the formation of an image. For that reason, in the case of a minute image, when even a slight vibration is generated during the operation, an image with good image quality is not reproduced. Furthermore, the radiation amount of exposure energy must be increased, and the radiation of a large amount of generated heat accompanying this needs to be considered. Also, the ink to be used for the inkjet recording mode is required to have various characteristics such as physical properties being hardly changed due to storage and precipitates and the like not being generated (solution stability), and clogging of a nozzle not being generated (discharge stability).
In general, as a method for enhancing the sensitivity to radiation rays in a radiation curable photopolymerizable composition, it is disclosed in Bruce M. Monroe, et al., Chemical Reviews, Vol. 93 (1993), pages 435 to 448, U.S. Pat. No. 4,134,813 and JP-A Nos. 1-253731 and 6-308727 that various polymerization initiation systems are useful. However, there is no example in which a polymerization initiation system which has sufficient sensitivity to scanning exposure and has sufficient storage stability and discharge stability has been employed in an inkjet recording ink.
For that reason, an ink composition suitably useful for inkjet recording, which is cured with high sensitivity even by radiation rays of a low output, is able to form an image with high image quality and has good storage stability and discharge stability, is earnestly desired.